2.8 Ambient Occlusion of World Background
For the transparent background you need to enable Transparent in the Film panel. If you want to enable or disable this option for specific frames:


Move to the first frame in the timeline
Set the state of the Transparent checkbox (enable if you want the majority of the frames to be transparent, disable if you want most frames without transparency)
Hover over the checkbox with your mouse cursor and I to insert a keyframe.


Repeat the process with other frames of the animation where you want the opposite setting for the transparency. The keyframe setting will be applied until the next frame with a keyframe, which means for disabling transparency for one frame only, you will need to activate it again in the next one.

Make sure that the file format you're using supports alpha channels.





In order to remove the green lighting from the project, you will need to change the color or texture of the World. Ambient Occlusion is also in the World tab, if you still want to use it.